The present application relates to amplification and control of laser pulses in fiber laser devices.
Chirped pulse amplification (CPA) fiber lasers are a type of compact and economical laser device that can generate microjoule-level sub-picosecond laser pulses. Unlike traditional pulse laser systems, CPA fiber lasers do not need high-voltages and bulky water-cooling system. CPA fiber lasers are robust and simple to use, and can be implemented in turn-key designs that provide femtosecond pulses straight out-of-the box. Laser pulses produced by CPA fiber lasers can produce a thermal ablation in most materials, which makes it ideal for micro-machining and precision surgeries.
Pulsed laser devices have been used in eye surgical applications, with different types of eye surgeries having different requirements on the laser pulses. Corneal surgery (used in LASIK) and corneal transplant require low energy (˜1-5 μJ) and high repetition rate (200 KHz-1 MHz) laser pulses for smooth and fast cuts. Lens fragmentation in cataract surgery, on the other hand, requires higher energy laser pulses (>15 μJ) in loosely focused beams to achieve deeper cuts. Since laser device contributes to the majority of the cost in a laser surgery system, there is a long-felt economic need for a CPA fiber laser that can be used in both LASIK and cataract surgery applications.
The effectiveness of laser-pulse ablation depends on the peak power and the repetition rate of the laser pulses. The peak power in a femtosecond laser pulse provides the energy intensity necessary to ionize the target material, thereby generating plasma which ablates the target material. The repetition rate of the laser pulses determines the speed of the ablation. The repetition rate also determines the energy per pulse because the average power of a CPA fiber laser is limited by a maximum system capability and a safety limit for the ablation target (e.g. the eye).
Thus, for a given average power, peak pulse power and repetition rate trade off each other in a CPA fiber laser device: a lower repetition rate is accompanied by a higher energy per pulse, and vice-versa. As a result, conventional CPA fiber lasers generate either high-repetition-rate low-energy (HRLE) or low-repetition-rate high-energy (LRHE) pulses; but they cannot achieve high repetition rate and high peak pulse power simultaneously.
The peak power in ultra short (picoseconds or shorter) laser pulses usually does not linearly scale with the pulse energy per laser pulse in a fiber laser device. This non-linearity is a result of nonlinear interactions between the electric field and the dielectric glass medium in the tight confinement and long interaction length in optical fibers. In fiber-based CPA lasers, the impact of the nonlinear interaction is minimized by temporal and spatial stretching. In temporal stretching, the ultra short laser pulses are stretched prior to amplification, and recompressed after amplification to regain high peak power in the ultra short laser pulses. On the other hand, spatial stretching employs a large mode area (LMA) fiber in order to reduce the optical energy intensity in the fiber.
Unfortunately, limitations exist to both temporal and spatial stretching: temporal stretching is limited by the size of the compressor; the spatial stretching is limited by the existence of higher-order-modes in the LMA fiber. These limitations degrade temporal and spatial qualities of the laser pulses, resulting in incomplete removal of nonlinear interactions at the required high pulse energies. Self-phase-modulation (SPM), the lowest-order of nonlinear interaction, can be observed at the pulse energy level of tens of μJ. As the peak pulse power is increased, nonlinear phase due to the SPM is accumulated in a laser pulse. The nonlinear phase, which is quantified by the B-integral, needs to be compensated in order to achieve laser pulses of the shortest possible duration and highest possible peak power.
There is therefore a need for fiber laser devices that can output laser pulses with repetition rate and energy as required by above described applications, while maintaining spatial and temporal quality.